Joule heating

Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat.

Joule's first law, also known as the Joule–Lenz law,[1] states that the power of heating generated by an electrical conductor is proportional to the product of its resistance and the square of the current:

P∝I2⋅R{\displaystyle P\propto I^{2}\cdot R}

Joule heating affects the whole electric conductor, unlike the Peltier effect which transfers heat from one electrical junction to another.

Joule heating is caused by interactions between the moving particles that form the current (usually, but not always, electrons) and the atomicions that make up the body of the conductor. Charged particles in an electric circuit are accelerated by an electric field and have electrostatic potential energy. When the charged particles collide with ions in the conductor, the particles are scattered and so their motion becomes random and therefore thermal, increasing the temperature of the system as they continue to move through the circuit. Some kinetic energy is lost in these collisions however the drift velocities of these particles is of the order of mm/h and so kinetic energy loss is negligible and almost all kinetic energy comes from thermal motion. For example, carbon nanotubes have been found to emit infrared, visible, and ultraviolet radiation and reach temperatures over 4000 K when exposed to microwaves. Although a "perfect" carbon nanotube is theoretically capable of ballistic conduction, defects in nanotubes result in Joule heating when exposed to microwave fields.[3]

Joule heating is referred to as ohmic heating or resistive heating because of its relationship to Ohm's Law. It forms the basis for the large number of practical applications involving electric heating. However, in applications where heating is an unwanted by-product of current use (e.g., load losses in electrical transformers) the diversion of energy is often referred to as resistive loss. The use of high voltages in electric power transmission systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents. The ring circuits, or ring mains, used in UK homes are another example, where power is delivered to outlets at lower currents, thus reducing Joule heating in the wires. Joule heating does not occur in superconducting materials, as these materials have zero electrical resistance in the superconducting state.

In plasma physics, the Joule heating often needs to be calculated at a particular location in space. The differential form of the Joule heating equation gives the power per unit volume.

dP/dV=J⋅E{\displaystyle dP/dV=\mathbf {J} \cdot \mathbf {E} }

Here, J{\displaystyle \mathbf {J} } is the current density, and E{\displaystyle \mathbf {E} } is the electric field. For a neutral plasma not in magnetic field and with a conductivity σ{\displaystyle \sigma }, J=σE{\displaystyle \mathbf {J} =\sigma \mathbf {E} } and therefore

In electric power transmission, high voltage is used to reduce Joule heating of the overhead power lines. The valuable electric energy is intended to be used by consumers, not for heating the power lines. Therefore, this Joule heating is referred to as a type of transmission loss.

A given quantity of electric power can be transmitted through a transmission line either at low voltage and high current, or with a higher voltage and lower current. Transformers can convert a high transmission voltage to a lower voltage for use by customer loads. Since the power lost in the wires is proportional to the conductor resistance and the square of the current[clarification needed], using low current at high voltage reduces the loss in the conductors due to Joule heating (or alternatively allows smaller conductors to be used for the same relative loss).

Joule-heating or resistive-heating is used in uncountable number of gadgets and industrial process. The part of the gadget, that converts electricity into heat through the process of resistive or Joule heating is called a heating element.

Electric fuses rely on the fact that if enough current flows, enough heat will be generated to melt the fuse wire.

Electronic cigarettes usually work by Joule heating, vaporizing propylene glycol and vegetable glycerine.

Thermistors are resistors whose resistance changes when the temperature changes. These are sometimes used in conjunction with Joule heating: If a large current is sent through the thermistor, the device's temperature rises and therefore its resistance changes. If the device has a positive temperature coefficient of resistance (PTC), the rise in temperature will cause a drop in current, making the device useful in a circuit-protection role similar to fuses, or for feedback in circuits, or for many other purposes. In general, self-heating can turn a resistor into a nonlinear and hysteretic circuit element. For more details see thermistor#Self-heating effects.

Food processing equipment may make use of Joule heating in food production. In this case, the food material serves as an electrical resistor, and heat is released internally.[5]

As a heating technology, Joule heating has a coefficient of performance of 1.0, meaning that every joule of electrical energy supplied produces one joule of heat. In contrast, a heat pump can have a coefficient of more than 1.0 since it moves additional thermal energy from the environment to the heated item.

The definition of the efficiency of a heating process requires defining the boundaries of the system to be considered. When heating a building, the overall efficiency is different when considering heating effect per unit of electric energy delivered on the customer's side of the meter, compared to the overall efficiency when also considering the losses in the power plant and transmission of power.

^R.J.Oosterbaan, J.Boonstra and K.V.G.K.Rao (1996). The energy balance of groundwater flow(PDF). In: V.P.Singh and B.Kumar (eds.), Subsurface-Water Hydrology, Vol.2 of the Proceedings of the International Conference on Hydrology and Water Resources, New Delhi, India. Kluwer Academic Publishers, Dordrecht, The Netherlands. pp. 153–160. ISBN978-0-7923-3651-8.